Gottwald، S. (2010). "An early approach toward graded identity and graded membership in set theory". Fuzzy Sets and Systems. ج. 161 ع. 18: 2369–2379. DOI:10.1016/j.fss.2009.12.005.
Klaua, D. (1965) Über einen Ansatz zur mehrwertigen Mengenlehre. Monatsb. Deutsch. Akad. Wiss. Berlin 7, 859–876. A recent in-depth analysis of this paper has been provided by Gottwald، S. (2010). "An early approach toward graded identity and graded membership in set theory". Fuzzy Sets and Systems. ج. 161 ع. 18: 2369–2379. DOI:10.1016/j.fss.2009.12.005.